The agency has also argued that daily delivery rates submitted to the courts do not accurately reflect performance, as the scores fluctuate based on day and mail volume. However, a Postal Service spokesman said he could not provide weekly figures by region.
The Postal Service has blamed the pandemic and labor shortages for some of the poor rates. As coronavirus cases surge across the country, the Detroit district reported just 78 percent of its employees were available to work, while Central Pennsylvania reported 84 percent, according to a court filing on Friday.
“This has been one of the things that I’ve been really worried about this year,” said Tammy Patrick, a former Arizona elections administrator and a senior adviser at Democracy Fund, a research and advocacy group. “When you get closer and closer to Election Day, your options tend to run out.”
Keith Combs, the president of the Detroit chapter of the American Postal Workers Union, said the post office was still recovering from operational cutbacks imposed this summer by Louis DeJoy, the postmaster general, while it continued to deal with a shortage of employees.
Backups in mail delivery were better than they once were, but customers were still experiencing delays, Mr. Combs said. The Postal Service has been careful to try to shield ballots from the slowdowns, but some might slip through the cracks, he added.
Michigan and Pennsylvania are far from the only places to report problems. Lakeland, the Postal Service district that covers most of Wisconsin and Northern Illinois, has consistently fallen below the national average, along with Atlanta. The Gulf Atlantic district, which includes parts of Georgia and Florida, has also performed below the national average most days this week.
Other swing states have not experienced delays that are as widespread. Certain postal districts in Florida, including Suncoast and South Florida, have mostly had ballot delivery to election officials at rates above 90 percent this week. With the exception of Friday and Saturday, the rate of on-time mail delivery in Arizona has stayed above 97 percent.